The Complete Guide to Satellite Fishing Charts

A satellite fishing chart is built from four things: a sensor, a satellite carrying it, a processing step that turns raw measurements into a picture, and a delay between when the water was actually measured and when you see it. This guide is the map of that whole chain: what each piece is, why it matters, and which page in this library goes deeper on each one.

The two products: pass and composite

Every SST or chlorophyll chart you'll ever look at is one of two things. A pass is a single satellite's single overflight: real measurements, real holes where cloud got in the way, usually a few hours old. A composite is a processed analysis product, built once a day, that blends multiple sources and fills the gaps a lone pass would have: no holes, but a day or more old and, for SST, an estimate rather than a direct reading everywhere it fills a gap. Chlorofishy's SST composite is NASA's GHRSST MUR Level 4 analysis, 1km resolution, published on a measured lag of about 34 hours; the chlorophyll composite is a VIIRS Level-3 daily product from NASA's Ocean Biology group, 4km resolution, with a measured latency of roughly 12 to 14 hours. Full explanation: raw pass vs. composite SST. For how the composite gets built without simply inventing data for cloudy areas, see how cloud-free SST charts work and is cloud-free SST real or interpolated. And when a chart shows nothing at all where you're looking, that has a specific, non-mysterious cause. See why your chart is blank.

The sensors: not one satellite, several

“Satellite data” is not one thing. Chlorofishy alone pulls from VIIRS aboard three separate satellites (Suomi-NPP, NOAA-20, NOAA-21), MODIS aboard Aqua, OLCI aboard the two Sentinel-3 satellites, and AMSR2 aboard GCOM-W: four kinds of instrument on eight spacecraft, each with a different resolution, a different revisit schedule, and in AMSR2's case an entirely different physical principle (microwave instead of visible/infrared light). Every fishing app draws from some subset of this same public pool, and which subset it uses is the single biggest reason two apps can show you different pictures of the same water on the same day. See do all fishing apps use the same satellite data? if you take away only one page from this guide. For the sensor lineup by name, see VIIRS vs. MODIS vs. AVHRR and microwave SST. For how often any given sensor actually flies back over the same patch of ocean, see satellite revisit times.

Resolution: what a pixel actually covers

Resolution ranges from VIIRS's ~750m chlorophyll pixels down to AMSR2's ~25km microwave footprint: a 1,000x difference in area per pixel between the sharpest and coarsest sensor Chlorofishy ingests. No processing step ever stretches a coarser product to look sharper than its source data; a 1km MUR composite pixel is genuinely 1km, not upsampled from something coarser. Full breakdown by product: native resolution, explained. Resolution is also the biggest reason two sensors looking at the same water minutes apart can disagree at the pixel level. See why satellites disagree.

Latency: how old is “live”

A raw satellite pass typically reaches Chlorofishy a few hours after capture. The SST composite runs on a measured ~34-hour publish lag; the chlorophyll composite runs at roughly 12 to 14 hours. Neither number is a guess: both come from a 13-day freshness audit against the live pipeline. What that means in practice, and how to check the age of any specific chart you're looking at, is covered in how old is your SST data?

What the temperature reading actually measures

A satellite doesn't measure water temperature the way a boat gauge does. It measures the top few micrometers of the surface (or, for a foundation product like MUR, a modeled temperature just below the daily heating/cooling cycle), not the water a few feet down where your intake sits. That distinction, and why the two numbers converge at night and in wind but can diverge sharply on a calm, sunny afternoon, is covered in why doesn't my boat's gauge match the chart?

Reading the ocean once you have the chart

Once you can trust what a chart is actually showing, the second half of this library is about what the ocean itself is doing. Temperature breaks and chlorophyll edges are where two different water masses meet, and where predators concentrate along the seam. Upwelling explains why cold water is often the productive water, not the water to avoid. Eddies and the thermocline cover the structure below the surface that a 2D chart can only hint at. Chlorophyll numbers that mean something is the deepest page in this cluster: what the actual mg/m³ values look like on the chart's fixed scale, and why the gradient between values matters more than any single reading. Secchi depth and reading water color connect the chlorophyll number to what you'll actually see over the rail. And combining SST and chlorophyll is the workflow that ties the whole guide together, using both layers at once instead of either alone.

Glossary

If you just need a definition, not the explanation, the glossary covers the eight terms that come up most: SST, temperature break, chlorophyll edge, Secchi depth, composite, satellite pass, upwelling, and eddy.

Seeing it on real water

Every fact in this guide is demonstrable on the live map, not just described. Chlorofishy's data sources page lists every sensor with its resolution and latency; the map itself shows the pass/composite label, capture time, and satellite name on every layer you open, on any of 38 regions worldwide. Start with the SST map or the chlorophyll map for the region you fish, or see pricing for what a plan includes beyond the free composite view. Want to see how Chlorofishy stacks up against the named vendors in this category? See Compare.